1. Field of the Invention
This invention is directed to optical amplification in communication networks and more particularly to a highly scalable modular optical amplifier based subsystem.
2. Background Art
To achieve long haul optical transmission, regenerators (repeaters) and/or optical amplifiers are deployed along the optical transmission line in multiple locations, for boosting the signal on the fiber. For systems operating at data rates of GBps, regenerator sites could be spaced in the range between 35 to 80 Km, depending on the wavelength chosen for transmission. The distance between optical amplifiers may be almost doubled, being in the range between 80 to 160 km.
Optical amplifiers are based on a length of Erbium doped fiber (active fiber) pumped with light of a certain wavelength to amplify the optical signal passing through the amplifier. The active fiber is spliced in the optical fiber. An important element is the WDM coupler, which performs the function of coupling the pump source lager wavelength to the Erbium doped fiber. Optical amplifiers may also be bidirectional, in which case they use a pump for each direction of transmission, with the respective WDM couplers. Optical isolators are also used internal to an optical amplifier, for reducing reflections generated at the points of fiber discontinuities, such as couplers, splitters, etc.
Optical amplifiers are favoured in long-distance systems over electrical repeaters not only because they allow for longer distances between the modules, and can be easily spliced into the fiber transmission link, but more importantly, because they do not require optical/electrical and electrical/optical conversion. An optical amplifier can amplify multiband/multichannel optical signals without demultiplexing them, thereby avoiding the costs of multiple optical receivers, multiple regeneration circuits and multiple optical transmitters. Also, they amplify whatever bit rate comes down the fiber. Even if the transmission rate is boosted, the device will not need to be replaced.
Current optical amplifiers are equipped with power monitors which control the pump based on measurements of the output and sometimes input signals. The measurement is effected by providing an optical tap coupler on the respective output and input optical signal and diverting a fraction (generally 3-5%) of the respective input and output signals to the monitor.
The 80 km limitation can be extended with the introduction of external modulation and use of dispersion shifted optical. WDM and dense WDM (DWDM) technologies reduce the strands of optical fiber cable needed to establish a communication link, and provide manifold capacity expansion on existing fiber links. In addition, the advances in fiber technology now permit optical amplifiers to work not only in the conventional band (C-band) of 1530-1563 nm, but also in an extended band (E-band) of 1570-1603 nm.
The number of amplifiers required for working and protection spans, the type of the optical amplifiers, and the number of wavelengths carried within the system are significant issues must be considered when designing multiband/multichannel transmission systems. As the optical amplifiers evolve in performance and functionality, so does their cost. Evolution of the network, e.g. in terms of bandwidth growth must also be taken into consideration. Currently, network providers use over-performing optical amplifiers than necessary at the first stage of network deployment for allowing for future growth.
The optical amplifiers available on the market accommodate up to 16 bands bidirectionally. These amplifiers are exclusively for bidirectional or unidirectional systems and are relatively inflexible to create various complex amplifier topologies. There are three types of optical amplifiers: post-amplifiers that connect to a transmitter to boost the output power; line amplifiers connected along a route between the transmitter and the receiver, and pre-amplifiers that improve the sensitivity of optical receivers. These different types of amplifiers provide different output powers, use different input power levels, and generally have different noise figure requirements. Being stand-alone units, they allow the network with little opportunity for growth or scalability, in that they must be replaced whenever the demand for bandwidth increases.
It is an object of the invention to provide a highly scalable modular optical amplifier based subsystem, which solves totally or in part the drawbacks of the prior art optical amplifiers.
It is another object of the invention to provide a highly versatile, scalable and modular family of optical amplifier building blocks that can be arranged in a variety of ways to produce both unidirectional and bidirectional topologies.
The building blocks or modules of the optical amplifier architecture are intended to operate in a modular manner exploiting the entire conventional Erbium gain window (1530 nm-1563 nm) as well as the extended Erbium band (1570 nm-1603 nm). Used together, this set, or family of products can produce optical amplifier topologies which can either be unidirectional or bidirectional, which also offer scalability with respect to the number of wavelengths deployed. The modules or building blocks are compatible with the current Northern Telecom Limited S/DMS TransportNode(trademark) products, and could be mapped into the existing shelves. It is expected that the equipping restrictions, mostly surrounding the OSC circuit pack, need to be employed due to hardware or software limitations, or to simplify system operation and verification. These restrictions will be documented in the equipping rules for the appropriate S/DMS TransportNode OC-192 releases.
According to one aspect of the invention, there is provided an optical amplification system comprising, a dual optical amplifier building block for bidirectional amplification of a plurality of optical channels propagating along a first and a second transmission line, an optical service channel (OSC) building block operatively connected to said dual optical amplifier building block for transmitting and receiving service information over a first and a second service channel, and a first Raman source of optical power connected at the input of said dual optical amplifier on said first transmission line for back pumping light.
According to a further aspect of the invention, there is also provided, an optical amplification system comprising, a dual optical amplifier building block for bidirectional line amplification of a plurality of optical channels propagating along a first and a second transmission line, an optical service channel (OSC) building block operatively connected to said dual optical amplifier building block for transmitting and receiving service information over a first and a second service channel, a first booster optical amplifier building block connected on said first transmission line at a first output of said dual optical amplifier building block, a second booster optical amplifier building block connected on said second transmission line at a second output of said dual optical amplifier building block, a grid-1 filter, connected between said first output and said first booster optical amplifier building block, a grid-2 filter, connected between said second output and said second booster optical amplifier building block, a first Raman source of optical power connected at the input of said dual optical amplifier on said first transmission line for back pumping light, and a second Raman source of optical power connected at the input of said dual optical amplifier on said second transmission line for back pumping light.
The invention further includes an optical amplification system comprising, an optical multiplexer for multiplexing a plurality of optical signals received over a plurality of input transmission lines and providing a forward multichannel optical signal, a dual optical amplifier building block for amplifying said forward multichannel optical signal and amplifying a reverse multichannel optical signal, an optical demultiplexer for receiving said reverse multichannel optical signal and separating same into a plurality of optical channels for transmission over a plurality of transmission lines, an optical service channel (OSC) building block operatively connected to said dual optical amplifier building block for transmitting and receiving service information over an optical service channel; a first Raman source of optical power connected at the input of said dual optical amplifier on said first transmission line for back pumping light, and a second Raman source of optical power connected at the input of said dual optical amplifier on said second transmission line for back pumping light.
The main advantage of the scalable and modular architecture according to the invention is the ability to provide a choice of optical amplifier architectures that may be adapted to the current need of the optical network and that scales as the bandwidth demand grows.